Normal Incidence Reflection Coefficient Research Articles

Absolute reflection coefficients from deep Seismic reflections

Many deep Seismic reflection profiles show bright sub-horizontal layered reflections in the lower continental crust. Accurate estimates are obtained for normal incidence reflection coefficients for the brightest of these reflections by using a marine Seismic acquisition system that has been calibrated using deep-water sea-bottom multiples. Minimum bounds are placed upon these reflection coefficients by assuming minimum losses due to scattering, anelastic attenuation and geometric spreading. Reflection coefficients of around 0.1 are obtained from the layered lower crust and around 0.15 from the Moho. It is argued that constructive interference between successive reflections from a layered sequence of reflectors is often unimportant, consequently these coefficients can be used to place useful limits on the absolute acoustic impedance contrasts present at depth.

Amplitude‐versus‐offset variations in gas sands

Seismic reflections from gas sands exhibit a wide range of amplitude‐versus‐offset (AVO) characteristics. The two factors that most strongly determine the AVO behavior of a gas‐sand reflection are the normal incidence reflection coefficient [Formula: see text] and the contrast in Poisson’s ratio at the reflector. Of these two factors, [Formula: see text] is the least constrained. Based on their AVO characteristics, gas‐sand reflectors can be grouped into three classes defined in terms of [Formula: see text] at the top of the gas sand. Class 1 gas sands have higher impedance than the encasing shale with relatively large positive values for [Formula: see text]. Class 2 gas sands have nearly the same impedance as the encasing shale and are characterized by values of [Formula: see text] near zero. Class 3 sands have lower impedance than the encasing shale with negative, large magnitude values for [Formula: see text]. Each of these sand classes has a distinct AVO characteristic. An example of a gas sand from each of the three classes is presented in the paper. The Class 1 example involves a Hartshorn channel sand from the Arkoma Basin. The Class 2 example considers a Miocene gas sand from the Brazos offshore area of the Gulf of Mexico. The Class 3 example is a Pliocene gas sand from the High Island offshore area of the Gulf of Mexico.

The determination of acoustic reflection coefficients by using cepstral techniques, I: Experimental procedures and measurements of polyurethane foam

The authors have previously outlined a transient free field technique, based on cepstral analysis, for the measurement of acoustic reflection coefficients. In this paper are described laboratory acoustical measurements of the normal incidence reflection coefficient of an absorbent material: emphasis is placed on practical aspects of the technique. Specifically, the origin of extraction noise, which distorts the reflector impulse response as it appears in the power cepstrum, is discussed and means of reducing it are described and implemented. Secondly, a means of identifying and removing the time delay introduced when the reflector impulse response is copied from the cepstrum is described; this procedure eliminates the need for highly accurate measurements of path length difference. The absorbent material tested was a commercial partially reticulated polyurethane foam. Bonded to one side of the foam was an impermeable polyurethane membrane, and the foam was measured in two configurations: first with its film covered face uppermost, then with its uncovered face uppermost. The broad frequency range of the measurements made possible by the cepstral technique has given a good picture of the properties of this material. These results will be considered in detail in a subsequent publication.

EQUATIONS D'ONDE ET MODELES *

AbstractThe method of finite differences is applied to the computation of multi‐dimensional synthetic seismograms. This paper gives a study of the mathematical and numerical formulations of the problem, the boundary conditions, the convergence conditions and how to simulate the source in both one solid or a liquid. It is shown that the numerical formulation chosen is valid both for direct and inverse problems (i.e. for modeling and migration). This formulation makes it possible to use the normal incidence reflection coefficients for P and S waves, whether they travel horizontally or vertically. The examples shown have been chosen on purpose in order to be easily interpreted. They do not give a full idea of the possibilities of the algorithm which allows to consider non‐planar interfaces, except close to the vertical axis.